1. Field of the Invention
This invention relates to axial flow rotary machines, and more particularly to tip shrouds for compression stages of gas turbine engines.
2. Description of the Prior Art
The concepts of the present invention are described with respect to the fan stage of a turbofan, gas turbine engine. Although the concepts disclosed have applicability in other compression stages, most of the prior research and development in this area has been in relation to fan stages. In such fan stages a plurality of rotor blades extend radially outward from a rotor shaft across a flow path for the working medium gases. An engine case encloses the fan blades. A shroud housed in the engine case circumscribes the tips of the blades.
The aerodynamic efficiency of the fan stage is materially effected by the clearance between the tips of the blades and the corresponding seal land. As the clearance is increased, substantial amounts of working medium gases leak circumferentially over the tips of the blades from the pressure sides to the suction sides of the airfoils. Additionally, amounts of medium gases leak axially over the tips from the downstream end to the upstream end of the airfoils.
The historic approach in controlling leakage has been to minimize the clearance dimension between the tips and the corresponding shroud at the design operating condition. Such, however, is not an easy task as during operation of engine the relative radial distance between the tips of the blades and the corresponding shrouds varies substantially. For example, as the rotor is turned to speed, centrifugally generated forces cause the tips of the rotor blades to be displaced radially outward toward the corresponding shroud. Collaterally, flexure of the rotor and of the engine case causes relative displacements between blade tips and the corresponding shroud. Sufficient initial clearance between the tips and the shroud must be provided to prevent destructive interference during this initial period.
In an effort to avoid unduly large initial clearances many modern engines utilize abradable shrouds in which the airfoil tips are allowed to wear into the shrouds during transient excursions. U.S. Pat. Nos. 3,519,282 to Davis entitled "Abradable Material Seal"; 3,817,719 to Schilke et al entitled "High Temperature Abradable Material and Method of Preparing the Same"; and 3,918,925 to McComas entitled "Abradable Seal" are representative of such shrouds and their methods of manufacture. Accordingly, by such embodiments the clearance over the airfoil tips becomes the minimum clearance that will accommodate rotor excursions.
In addition to avoiding large initial clearances many modern engines employ porous shrouds such as those described in U.S. Pat. Nos. 3,580,692 to Mikolojczak entitled "Seal Construction"; and 3,843,278 to Torell entitled "Abradable Seal Construction". Such constructions are thought to improve engine performance by reducing the depth of the flow boundary layer adjacent to the suction side surfaces of the airfoils.
Other techniques for reducing leakage across the blade tips have been investigated. One such technique relevant to the presently disclosed concepts is reported in NASA Technical Memorandum X-472 by Kofskey entitled "Experimental Investigation of Three Tip-Clearance Configurations Over a Range of Tip Clearance Using a Single-Stage Turbine of High Hub to Tip-Radius Ratio". Specifically, the "recessed casing" reported in the memorandum and illustrated in FIG. 3(b) is of interest. In accordance with the Kofskey teaching improved efficiency over conventional, smooth wall shrouds is obtainable by submerging the tips of turbine blades into a recess in the corresponding shroud. A comparison of smooth wall and recessed casing efficiencies is shown in FIG. 8 of Kofskey. Also shown in Kofskey is a comparison in FIG. 6 between a recessed casing in which the blade tips are submerged and a recessed casing in which the blade tips run line on line with the flow path wall. The tests show the submerged construction to be markedly superior over the line on line construction by several percentage points in efficiency.
Notwithstanding the advanced state of the shroud art, manufacturers of rotary machines are devoting substantial resources in this area to the improvement of machine efficiencies and operating characteristics.